Method and apparatus for regulating electrical power output of a fuel cell system

ABSTRACT

A method and apparatus are provided for regulating electrical power output of a fuel cell system comprising a fuel processing system, a system for supplying an oxidant stream, and a fuel cell to supply power. In a reformer of the fuel processing system a supply fuel is used to produce hydrogen-rich gas to be supplied to the fuel cell. The fuel cell system also contains a controller to set the mass flow of the supply fuel to the fuel processing system and the mass flow of the oxidant stream to the fuel cell, whereby the mass flow of the oxidant stream can be set in dependence on the dynamic response of the fuel processing system and/or the mass flow of the hydrogen-rich gas can be set in dependence on the dynamic response of the system for supplying the oxidant stream.

BACKGROUND

1. Field of Invention

The invention relates to a method and apparatus for regulatingelectrical power output of a fuel cell system.

2. Description of the Related Art

U.S. Pat. No. 5,432,710 A describes a power supply system that containsa fuel cell and a controller. The controller regulates the systems andsubsystems of the power supply system by minimizing a cost function inthe form of an algebraic equation. This cost function takes into accountthe power demand of the load, the power demand of the system itself, andthe exhaust gases. In dependence on the cost function, the controllersets the mass flow of an oxidant stream and the mass flow of ahydrogen-rich gas for the fuel cell unit, and the mass flow of a fuelfor the reformer.

JP 59-11270 describes a fuel cell system comprising differentialpressure controlling valves for controlling a differential pressurebetween the pressures of supplied oxygen and hydrogen. The hydrogen issupplied from a hydrogen tank. The differential pressure control valvesare connected to each other by a link mechanism. Thus, the operation ofone pressure control valve is controlled in dependence on the operationof the respective other differential pressure control valve.

JP 5911273 describes a similar fuel cell system with two pressurecontrol valves which are controlled in dependence from the respectiveother pressure control valve.

EP 1 207 578 A2, which is not prepublished, describes a fuel cell systemwherein the hydrogen is supplied from a high pressure hydrogen tank tothe fuel cell system. In this document it is mentioned that pressurehydrogen supplied from the high pressure hydrogen tank to the fuel cellis based on the air pressure supplied from the air supply side. Thatmeans that the pressure of the hydrogen supplied to the fuel cell systemis controlled in dependence on the air pressure supply from the airsupply side.

There is a need for improvement in regulating power supply from a fuelcell to a load. The present invention addresses this need and providesfurther related advantages.

BRIEF SUMMARY OF THE INVENTION

The invention provides a method of operating a fuel cell system with thefeatures according to claim 1. The fuel cell system comprises a fuelcell with an anode stream passage and a cathode stream passage, an anodesupply system for supplying a hydrogen-rich gas to the anode streampassage, a cathode supply system for supplying an oxidant stream to thecathode stream passage and a controller for operating the anode andcathode supply systems.

The method comprises operating one supply system in dependence on thedynamic response of the other supply system. The hydrogen-rich gas massflow to the anode stream passage may be set in dependence on the dynamicresponse of the cathode supply system and/or the oxidant stream massflow to the cathode stream passage may be set in dependence on thedynamic response of the anode supply system.

The invention also provides a method of operating a fuel cell comprisinga fuel processing system for converting a supply fuel into ahydrogen-rich gas. In such an embodiment, the supply fuel mass flow tothe fuel processing system may be set in dependence on the dynamicresponse of the cathode supply system and/or the oxidant stream massflow to the cathode stream passage may be set in dependence on thedynamic response of the fuel processing system.

In a further embodiment, the supply fuel mass flow to the fuelprocessing system may be set in dependence on the efficiency of the fuelprocessing system.

In a still further embodiment, the supply fuel mass flow to the fuelprocessing system may be set in dependence on the intrinsic consumptionof hydrogen-rich gas In the fuel processing system.

The Invention also provides a fuel cell system with the featuresaccording to claim 7. The fuel cell system comprises:

-   -   a) a fuel cell comprising an anode stream passage and a cathode        stream passage;    -   b) an anode supply system for supplying a hydrogen-rich gas to        the anode stream passage;    -   c) a cathode supply system for supplying an oxidant stream to        the cathode stream passage;    -   d) a controller for operating one supply system in dependence on        the dynamic response of the other supply system.

The anode supply system may comprise a fuel processing system forconverting a supply fuel into a hydrogen-rich gas.

Pursuant to the invention, the controller may set the hydrogen-rich gasmass flow to the anode stream passage In dependence on the dynamicresponse of the cathode supply system. Alternatively, the controller mayset the supply fuel mass flow to the fuel processing system independence on the dynamic response of the cathode supply system.

In an alternative embodiment, the controller may set the oxidant streammass flow to the cathode stream passage in dependence on the dynamicresponse of the anode supply system or in dependence on the dynamicresponse of the fuel processing system.

In a further alternative embodiment, the controller may set the supplyfuel mass flow to the fuel processing system in dependence on theefficiency of the fuel processing system.

In a still further alternative embodiment, the controller may set thesupply fuel mass flow to the fuel processing system in dependence on theintrinsic consumption of hydrogen-rich gas in the fuel processingsystem.

Many specific details of certain embodiments of the invention are setforth in the detailed description below to provide a thoroughunderstanding of such embodiments. One skilled in the art, however, willunderstand that the present invention may have additional embodiments,or may be practiced without several of the details described.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 shows a block diagram illustrating a method and apparatus forregulating the electrical power output of a fuel cell system accordingto the invention.

FIG. 2 shows in diagrams a, b, c, d, e, and f as a function of time, themass flow of a hydrogen-rich gas, the mass flow of an oxidant stream,and the output current of a fuel cell system that is operated withoutusing the method and apparatus according to the invention.

FIG. 3 shows in diagrams a, b, and c as a function of time, the currentdemand, the mass flow of a supply fuel, and the mass flow of ahydrogen-rich gas in a fuel cell system, with a proportional controllerwith derivative action in the supply branch for the hydrogen-rich gas.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

For the generation of electrical energy, a hydrogen-rich gas and anoxidant stream are supplied to a fuel cell through different supplylines. In fuel cell systems that are operated with a reformer, thesupply line for supplying a hydrogen-rich gas typically includes a fuelprocessing system with a reformer, in which a hydrogen-rich reformatestream is produced, and typically other fuel processing and/or gaspurification equipment, while the supply line for supplying an oxidantstream includes a separate oxidant supply system. This system forsupplying oxidant stream usually includes a compressor to control themass flow of the oxidant stream. In the method according to theinvention, these two supply lines can be operated inter-dependently.This means that when supplying oxidant stream, the system can take intoaccount the dynamic response or possible delays of the fuel processingsystem, and that the hydrogen-rich gas can be supplied in dependence onthe dynamic response or possible delays of the system for supplyingoxidant stream. This option for the supply paths to influence each otherimproves the regulation of the output of the fuel cell system and thusimproves the manner in which the overall fuel cell system provides thedesired power output.

Hydrogen-rich gas is produced in the fuel processing system from asupply fuel that usually contains a carbon- and hydrogen-rich medium,such as for example methanol. The consumption and loss of hydrogen-richgas in the fuel processing itself and the efficiency of the fuelprocessing system can be taken into account when determining therequired supply fuel quantity. This results in a more accuratedetermination of the required quantity of supply fuel to be delivered tothe fuel processor, and thus in a more precise output regulation.

What follows is a more detailed description, with the help of FIG. 1, ofa method and apparatus for regulating the electrical power output of afuel cell system according to the invention. The method and apparatusare particularly applicable when the fuel cell system comprises a fuelprocessing system 1 to provide a hydrogen-rich medium or gas, inaddition to a system 2 for supplying an oxidant stream, and a fuel cell3 to supply power to electrical loads. However, the method and apparatusare also applicable in a fuel cell system which does not have a fuelprocessor, in which a hydrogen-rich gas (for example, substantially purehydrogen) is supplied from a hydrogen storage tank or medium. The fuelcell 3 is thus supplied with a hydrogen-rich stream (for example, areformate stream or pure hydrogen) and an oxidant stream for example anoxygen-rich gas, such as air. Hydrogen-rich gas is typically producedfrom a supply fuel in a reformer (not shown) of the fuel processingsystem 1. This supply fuel may contain alcohols, ethers, esters,hydrocarbons, such as for example natural gas or gasoline, and/or anyother medium that can be used to produce hydrogen for the operation of afuel cell system. The preferred supply fuel is methanol. The system 2for supplying the oxidant stream comprises a compressor (which is notshown) to set the mass flow of the oxidant stream {dot over (m)}_(Air).The compressor is a parasitic electrical load that is provided withcurrent I_(Sys) by fuel cell 3. The fuel cell system contains acontroller 4 to adjust the mass flow of the supply fuel {dot over(m)}_(Fuel) directed to fuel processing system 1 and to adjust the massflow of the oxidant stream {dot over (m)}_(Air) directed to fuel cell 3.In the signal flow paths that are associated with the hydrogen-rich gasand the oxidant stream, the controller 4 is located upstream of the fuelprocessing system 1 and upstream of the system 2 for supplying theoxidant stream. In an advantageous manner, an arithmetic unit 8 islocated upstream of the controller 4 in the signal flow path 11, anduses the current or power demand I_(target) to generate a target valuefor the supply fuel quantity to be metered, or for the supply fuel massflow {dot over (m)}_(Fuel),. Analogously, an arithmetic unit 9 islocated upstream of the controller 4 in the signal flow path 12, anduses the current demand or power demand I_(target) to generate a targetvalue for the speed of the compressor (not shown) or for the mass flowof the oxidant stream {dot over (m)}_(Air). The controller 4 can beintegrated in a control device. Alternatively, the areas of thecontroller 4 that are only associated with one signal flow path 11, 12,may be integrated in separate control devices that are associatedexclusively with the signal flow paths 11, 12.

In accordance with the invention, the mass flow of the oxidant stream{dot over (m)}_(Air) directed to fuel cell 3 can be adjusted independence on the dynamic response of the fuel processing system 1. Bydynamic response of a system it is understood that a fundamentalcharacteristic of the system is that its response is time-dependent. Ifthe system can be represented by a differential equation or a differenceequation, then this equation can describe its dynamic behaviour. Forexample, in a linear differential equation, the eigenvalues and theeigenvectors determine the transient response of the system. Thetransient response is characterised by possible time delays, a certaindamping, possible overshoots, etc. Dead times are also a part of thedynamic response of a system.

Controller 4 contains a first filter unit 5, which is arranged in signalflow path 12. The parameters of filter unit 5 can be adjusted independence on the dynamic response of fuel processing system 1 (dot-dasharrow linking elements 1 and 5). The parameters can either be set usinga simulation model of fuel processing system 1, and/or using measuredvalues and/or calculated values of the mass flow of the hydrogen-richgas {dot over (m)}_(H2), since these values are also dependent on thedynamic response of fuel processing system 1. The term simulation modelrefers to a mathematical and/or physical model of a system. A physicalmodel can be obtained by describing the system using appropriatephysical laws. A mathematical model can be obtained by measuring theinput and output quantities of the system and by approximating theresponse of the system with the help of these measured quantities, forexample, using the method of least squares known in the literature.

Mathematical models that are suitable for this are, for example, linearand non-linear differential equations or difference equations,performance characteristics maps, neural networks, the ARX model knownfrom the English-language literature (autoregressive model withexogenous input variables), etc. The simulation model or the parametersof the simulation model can be generated prior to using the system orduring the operation of the system and can be adjusted after certaintime intervals to match the actual system behaviour (adaptive modellingor identification). Adaptive modelling makes it possible to integrateeffects such as ageing or changed operating points into the simulationmodel.

Normally, fuel processing system 1 is characterised by a dynamicresponse that is slower than that of system 2 for supplying the oxidantstream, since the dynamic response of the latter is predominantlydetermined by the dynamic response of the air supply compressor (notshown), which usually. is characterised by a comparatively rapid dynamicbehaviour. In order to be able to introduce into fuel cell 3 therequired amount of oxidant stream at the same time, and not prior to, asthe corresponding amount of hydrogen-rich gas, for example during a loadchange, filter unit 5 preferably contains a proportional controller witha selectable order time delay or a selectable order time delay element(a so-called PT_(x) element). The parameters of the time delay element,such as the time constants and the amplification factor, and the ordinalnumber, can then be chosen in dependence on the dynamic behaviour of thefuel processing system 1. Preferably, one uses a second order time delayelement (PT₂).

The supply fuel mass flow {dot over (m)}_(Fuel) can also be adjusted independence on the dynamic response of the system 2 for supplying theoxidant stream. The controller 4 is equipped with a second filter unit6, which is arranged in the signal flow path 11. The parameters of thissecond filter unit 6 can be set in dependence on the dynamic behaviourof the system 2 for supplying the oxidant stream (dashed arrow linkingelements 2 and 6).

The parameters can be adjusted using the simulation model for system 2for supplying the oxidant stream and/or using measured and/or calculatedvalues of the mass flow of the oxidant stream {dot over (m)}_(Air),since these values also are dependent on the dynamic response of thesupply system 2. Should fuel processing system 1 possess a fasterdynamic response than system 2 for supplying the oxidant stream or thecompressor (not shown), then second filter unit 6 preferably contains aproportional controller with a time delay of any desired order or aselectable order time delay element (as a so-called PT_(x) element). Theparameters of the time delay element, such as the time constants and theamplification factor, and the ordinal number, can then be chosen independence on the dynamic behaviour of the system 2 for supplying theoxidant stream or of the compressor (not shown).

FIG. 2 shows curves for the operation of a fuel cell system in two caseswhere the air mass flow and the hydrogen-rich gas mass flow are notcoordinated or interdependent. FIGS. 2 a-c illustrate an example of thestep responses of hydrogen mass flow (diagram a), air mass flow (diagramb), and current I_(avallable) (diagram c), as a function of time and inresponse to a sudden increase in current demand I_(target) at time zero,wherein the increased air mass flow is supplied to the fuel cell 3earlier than the increased hydrogen mass flow. The current I_(available)is the current I_(actual) that can be produced by fuel cell 3 minus acurrent I_(Sys), which is consumed by auxiliary equipment (parasiticloads), such as the compressor, a high-pressure compressor of the fuelprocessing system, or an air conditioning system. This means thatI_(available) represents the current that is available to power anexternal load, for example, for the propulsion of a vehicle. The stepresponse of the hydrogen mass flow and the air mass flow arerepresentative of the step responses of the mass flows of ahydrogen-rich and an oxidant stream {dot over (m)}_(H2),{dot over(m)}_(Air).

If there is a sudden power demand on the fuel cell system, the stepresponse of the hydrogen mass flow increases slowly until it reaches thesteady-state final value (diagram a). The step response of the air massflow reaches its steady-state limiting value much earlier, i.e. it has ahigher rate of increase (diagram b). Consequently, during the rise timeof the hydrogen mass flow, the current I_(available) at first dropssignificantly and subsequently increases until it reaches itssteady-state limiting value (diagram c). A comparison of diagrams a, b,and c shows that an unwanted current dip—due to the increased powerdemand of the com{dot over (p)}ressor (I_(Sys)), which arises tooearly—can occur during the time interval during which the fuel cell issupplied with an increased quantity of air before hydrogen. This statecan arise if the compressor is activated to increase its output tooearly or if the compressor speed is too high.

FIGS. 2 d-f illustrate an example of the step responses of hydrogen massflow (diagram d), air mass flow (diagram e), and current I_(available)(diagram f), as a function of time. Solid lines represent the valuesthat are the result of a sudden increase in current demand I_(target) attime zero if the hydrogen mass flow is supplied to the fuel cell earlierthan the air mass flow. The step responses of the hydrogen mass flow andthe air mass flow are representative of the step responses of the massflows of a hydrogen-rich and an oxidant stream {dot over(m+EE_(H2),m)}_(Air). In this case (diagram d), before reaching itssteady-state limiting value, the step response of the hydrogen mass flowrises faster than in diagram a. The step response of the air mass flowreaches its steady-state limiting value at a later time, i.e. itincreases more slowly (diagram e). The behaviour of the currentI_(avallable) corresponds to that of the air mass flow (diagram f).

The dotted line in diagram d shows the behaviour of a hydrogen massflow, were it to correspond to the behaviour of the air mass flow shownin diagram e. A comparison of diagram d with diagram e shows that theincreased hydrogen mass flow is made available to the fuel cell 3earlier than the increased air mass flow. This state can arise if thecompressor is activated late or if the compressor speed is too low. Thearea A between the solid line and the dotted line in diagram d is anindication of the unutilized dynamic response of the fuel processingsystem 1. The increased supply of hydrogen or of a hydrogen-rich gas—incomparison to the supply of air or an oxidant stream—can lead tooverheating of a catalytic burner (if present), which usually isconnected downstream of the fuel cell 3 to combust hydrogen contained inthe fuel cell exhaust gas. It can also result in decreased fuelefficiency.

Taking into account the dynamic response of the fuel processing system 1when supplying the oxidant stream and taking into account the dynamicresponse of the system 2 for supplying the oxidant stream or that of thecompressor (not shown) when supplying supply fuel to the fuel processingsystem 1, makes it possible to synchronize or co-ordinate the supply ofhydrogen-rich gas and the supply of the oxidant stream to the fuel cellunit, i.e. the appropriate volumes or mass flows of the two reactantsrequired by the load are supplied to the fuel cell simultaneously. Thisresults in a rapid, reliable, and safe supply of power and preventsunwanted current-and/or voltage dips, or unnecessary thermal loads onother system components, such as a catalytic burner.

In a further embodiment of the invention, measured and/or calculatedvalues of the mass flow of the hydrogen-rich gas {dot over (m)}_(H2) areprovided to the controller 4 as additional input variables.

Furthermore, measured and/or calculated values of the mass flow of theoxidant stream {dot over (m)}_(Air) can be provided to the controller 4as well. Preferably, the controller contains a multi-variablecontroller, which uses as input variables the current demand I_(target)or a target value for the amount of supply fuel and a target value forthe compressor speed, and which uses as controlled variables the massflows of the hydrogen-rich gas {dot over (m)}_(H2) and of the oxidantstream {dot over (m)}_(Air), and which uses as manipulated variable themetering of the supply fuel quantity and the activation of thecompressor. In order to be able to implement the current demandI_(target) as accurately as possible, the filter unit 6 can contain aproportional controller or a proportional controller with derivativeaction to compensate for the slow dynamic response of the fuelprocessing system. Preferably, the transfer function of the proportionalcontroller with derivative action is given by the Laplace Transformequation$K \cdot \left( {1 + \frac{T_{v} \cdot s}{1 + {T_{1} \cdot s}}} \right)$whereby s is a complex variable. K is an amplification factor, and T_(V)and T₁ are time constants of the proportional controller with derivativeaction, with the usual relation T₁+T_(V)>T₁. The value of theamplification factor is preferably equal to 1.

FIG. 3 shows the current demand I_(target) (diagram a), the supply fuelmass flow {dot over (m )}_(Fuel) (diagram b), and the mass flow of thehydrogen-rich gas {dot over (m)}_(H2) (diagram c), as a function oftime. The solid lines represent the signal in the case when aproportional controller with derivative action is used (as in anembodiment of the present invention), while the dotted lines representthe signal resulting when such a proportional controller with derivativeaction is not used.

The current demand that is shown in diagram a of FIG. 3 has the shape ofa step function, which rises suddenly at time t₀ and returns to itsoriginal value at time t₁. Diagram b shows the supply fuel mass flow{dot over (m)}_(Fuel) as a function of time, which typically resultsfrom the change in current demand I_(target) shown in diagram a. If aproportional controller with derivative action is used, then the supplyfuel mass flow {dot over (m)}_(Fuel) increases suddenly at time to andthen drops exponentially to its steady-state final value, which isnormally reached in the time interval t₀<t<t₁, whereby the value of thesupply fuel mass flow {dot over (m)}_(Fuel) at time t₀ is larger thanthe steady-state final value. In the remainder of this description, thisphenomenon will be referred to as overshoot. Time is represented by thevariable t. When I_(target) is decreased at time t₁, the supply fuelmass flow {dot over (m)}_(Fuel) drops suddenly at time t₁ and then risesexponentially to its steady-state final value, which is normally reachedat t>t₁, whereby the value of the supply fuel mass flow {dot over(m)}_(Fuel) at time to is smaller than the steady-state final value(solid line). If no proportional controller with derivative action isused, then the behaviour of the supply fuel mass flow {dot over(m)}_(Fuel) is analogous to that of the current demand I_(target). Thisis to say that at time t₀ the supply fuel mass flow increases suddenlyand suddenly returns to its original value at time t₁ (dotted line b₁).Diagram c shows the resulting step response of the mass flow of thehydrogen-rich gas {dot over (m)}_(H2).

If a proportional controller with derivative action is used, then themass flow of the hydrogen-rich gas {dot over (m)}_(H2) usually risesexponentially at time to until it reaches its steady-state final value,and subsequently at time t₁ drops exponentially back to its originalvalue (solid line). If no proportional controller with derivative actionis used then the mass flow of the hydrogen-rich gas behaves identically,but it takes longer to reach the final value after t₀ or the originalvalue after t₁, i.e. the transient response times are longer (dottedline c₁).

This means that if a proportional controller with derivative action ispresent, the mass flow of the hydrogen-rich gas {dot over (m)}_(H2)reacts more rapidly to changes of the current demand I_(target) or thesupply fuel mass flow {dot over (m)}_(Fuel). The higher the overshootduring the metering of the supply fuel, the faster the required quantityof hydrogen-rich gas can be provided for the fuel cell 3. A largeovershoot with a short transient response time is practical. The choiceof the time constants T_(V) and T₁ influences the shape, the amplitudeand/or the width of the overshoot of the supply fuel curve. This is aresult of the fact that the step response and the transfer function arerelated to each other by the Inverse Laplace Transform equation. Thus,the amplitude and/or width can be chosen, but this choice is limited byparasitic delays and/or component limits.

Since the fuel processing system 1 usually possesses a slower dynamicresponse than the system 2 for supplying the oxidant stream, it is ofadvantage to integrate a proportional controller with derivative actioninto the filter unit 6, and to also include a time delay element intothe filter unit 5. This compensates for a time delay of the fuelprocessing system 1 in the signal flow path 11 and balances the morerapid response characteristics of the system 2 for supplying the oxidantstream.

In a further embodiment of the invention, it is possible to take intoaccount—for the setting of the supply fuel mass flow {dot over(m)}_(Fuel)—an efficiency n of the reformer (not shown) or of the entirefuel processing system 1, and/or the intrinsic consumption ofhydrogen-rich gas by the fuel processing system 1 itself. This intrinsicconsumption of hydrogen-rich gas is typically the result of one or moregas purification stages, which are usually located down-stream of thereformer.

Because for purposes of gas purification one typically supplies a largeramount of an oxidant stream, such as air, than is actually required, soas to be able to reduce the concentration of CO (carbon monoxide) in thereformate stream, a loss of hydrogen-rich gas in the fuel processingsystem 1 occurs as a consequence. This can be taken into account asintrinsic consumption for supply fuel metering purposes. The addedoxygen-rich gas is referred to as AD, i.e. air dosage. The intrinsicconsumption and efficiency η usually depend on the amount of supply fueldosage or on the supply fuel mass flow {dot over (m)}_(Fuel) The airdosage AD and the efficiency η can preferably be determined by usingperformance characteristic maps, which use as their input variables thesupply fuel dosage or the supply fuel mass flow {dot over (m)}_(Fuel).With the help of Faraday's laws, it is possible to use the currentdemand I_(target) and the air dosage AD to calculate the required massflow of hydrogen-rich gas {dot over (m)}_(H2), from which one cancalculate the supply fuel mass flow {dot over (m)}_(Fuel) to be set bymeans of the following equation${{\overset{.}{m}}_{Fuel} = {M_{Fuel} \cdot \frac{1}{K} \cdot \frac{1}{\eta} \cdot \left( {{\frac{\lambda_{H2}^{FC} \cdot n_{z}}{F \cdot z} \cdot I_{target}} + {\frac{2 \cdot 0.21}{V_{0}} \cdot {AD}}} \right)}},$whereby M_(Fuel) is the molar mass of the supply fuel, n is the numberof fuel cells in the fuel cell unit, z is the valency of hydrogen (z=2),V₀ is the normal volume or molar volume of oxygen, and F is Faraday'sconstant. λ_(H2) ^(FC) represents the stoichiometric coefficient ofhydrogen in the reaction equation$\left. {{\lambda_{H2}^{FC} \cdot H_{2}} + {\frac{1}{2} \cdot \lambda_{O2}^{FC} \cdot O_{2}}}\rightarrow{{H_{2}O} + {\left( {\lambda_{H2}^{FC} - 1} \right) \cdot H_{2}} + {\frac{1}{2} \cdot \left( {\lambda_{O2}^{FC} - 1} \right) \cdot O_{2}}} \right.$and λ_(O2) ^(FC) represents the stoichiometric coefficient of oxygen inthe above reaction equation. The number 2 appears in the numerator ofthe second term inside the brackets because the fuel cell requires twohydrogen molecules H₂ for each oxygen molecule O₂ for the reaction tooccur. The number 0.21, which also appears in the numerator of thesecond term inside the brackets, represents the percentage of oxygen inair.

The factor k indicates how many moles of the hydrogen-rich gas can beproduced with one mole of supply fuel in an ideal fuel processing systemwith η equal to 100%. For example, if methanol is used as supply fuel,it is possible to produce 3 moles of hydrogen and k equals 3. Takinginto account the efficiency η and the air dosage AD of the fuelprocessing system 1 the reformer (not shown) allows a more accuratedosage of supply fuel, which results in a more accurate dosage of thehydrogen-rich gas. Block 7 in FIG. 1 represents the effect of the airdosage AD on the dosage of the hydrogen-rich gas.

In a further embodiment of the invention, output current I_(actual) isdetermined from the mass flow of the hydrogen-rich gas {dot over(m)}_(H2) and the mass flow of the oxidant stream {dot over (m)}_(Air)by means of a simulation model 10 that is based on the fuel cell 3. Theoutput current I_(actual) represents the maximum current that may berequested or consumed by the loads. The output current I_(actual) is theinstantaneous current that the fuel cell 3 can provide. In contrast, theactual measurable output current of the fuel cell unit, I_(FC),represents the current that is actually required and drawn by the loads.The simulation model can, for example, be a proportional controller witha time delay of any desired order or a time delay element of any desiredorder (a so-called PT_(x) element).

In a further embodiment of the invention, output current I_(actual) isdetermined from the current demand I_(target) by means of a secondsimulation model (not shown), which is based on the fuel cell 3, thefuel processing system 1, and the system 2 for supplying the oxidantstream. In this case as well, the output current I_(actual) representsthe maximum current that can be requested or consumed by the loads.

The output current I_(actual) is the instantaneous current that the fuelcell 3 can provide. This simulation model can for example be aproportional controller with a time delay of any desired order or a timedelay element of any desired order (a so-called PT_(x) element),preferably a fourth order time delay element (PT₄ element).

The use of simulation models to calculate the output current I_(actual)eliminates the need to use potentially expensive sensors in the fuelcell system. Moreover, the above-described simulation models providenoise-free signals as output signals. A suitable choice of appropriatesimulation models makes it possible to represent the above-mentionedsubsystems of the fuel cell system, e.g. the fuel cell 3, with anydesired accuracy.

It would be useful, if in a mobile device, such as a fuel cell vehicle,it would be possible to distinguish between the current that is requiredfor the propulsion of the vehicle and the current that is needed forother electrical systems or loads that are integrated into the vehicle,such as for example the compressor, a high-pressure compressor of thefuel processing system, or an air conditioning system. The electricalparasitic loads also include loads in the vehicle electrical system,such as in a 12V or a 42V vehicle electrical system. For improvedclarity, the first-mentioned current will be referred to as I_(Prop),while the latter current will be referred to as I_(Sys). The currentdemand I_(target) can be calculated as the sum of I_(Prop) and I_(Sys).From the current that has been measured and/or simulated or calculatedwith the help of the above-mentioned simulation models one candetermine—taking into account the current consumption I_(Sys) of theother electrical loads—an available current I_(available), which—in thecase of a vehicular application—is available for the propulsion of thevehicle by means of an electric motor. This allows a rapid and precisecontrol of the required current I_(Prop).

The principles disclosed herein apply to a sudden increase in currentdemand as well as to sudden current reduction in current demand, such aswhen the fuel cell system is shut down. In such a case, the fuelprocessing system 1 is typically not as quick in stopping the productionof hydrogen-rich gas, when compared to the system 2 stopping the supplyof oxidant stream. As outlined previously, the increased supply of thehydrogen-rich gas (when compared to the oxidant stream) can lead tooverheating of a catalytic burner located downstream of the fuel cell 3.The parameters of the time delay element could therefore vary dependingon whether the sudden load change is negative or positive. For example,the parameters of filter unit 5 could be selected to protect adownstream catalytic burner from overheating during shutdown.

While particular elements, embodiments and applications of the presentmethod and apparatus have been shown and described herein, it will beunderstood, of course, that the invention is not limited thereto sincemodifications may be made by those skilled in the art, particularly inlight of the foregoing teachings.

It is therefore contemplated by the appended claims to cover suchmodifications as incorporate those features which come within the scopeof the invention.

1. A method of operating a fuel cell system comprising a fuel cell (3)with an anode stream passage and a cathode stream passage, an anodesupply system (1) for supplying a hydrogen-rich gas to the anode streampassage, a cathode supply system (2) for supplying an oxidant stream tothe cathode stream passage and a controller (4) for operating the anodeand cathode supply systems (1, 2), the method comprising operating onesupply system in dependence on the dynamic response of the other supplysystem, characterized in that the fuel cell system comprises a fuelprocessing system (1) for converting a supply fuel into a hydrogen-richgas wherein the supply fuel mass flow ({dot over (m)}_(Fuel)) to thefuel processing system (1) is set in dependence on the dynamic responseof the cathode supply system (2)
 2. The method of claim 1, wherein thehydrogen-rich gas mass-flow ({dot over (m)}_(H2)) to the anode streampassage is set in dependence on the dynamic response of the cathodesupply system (2).
 3. The method of claim 1, wherein the oxidant streammass flow ({dot over (m)}_(Air)) to the cathode stream passage is set independence on the dynamic response of the anode supply system (1). 4.The method of claim 1, wherein the oxidant stream mass flow ({dot over(m)}_(H2)) to the cathode stream passage Is set in dependence on thedynamic response of the fuel processing system (1).
 5. The method ofclaim 1, wherein the supply fuel mass flow ({dot over (m)}_(Fuel)) tothe fuel processing system is set in dependence on the efficiency of thefuel processing system (1).
 6. The method of claim 1, wherein the supplyfuel mass flow ({dot over (m)}_(Fuel)) to the fuel processing system (1)is set in dependence on the intrinsic consumption of hydrogen-rich gasin the fuel processing system (1).
 7. A fuel cell system comprising: a)a fuel cell (3) comprising an anode stream passage and a cathode streampassage; b) an anode supply system (1) for supplying a hydrogen-rich gasto the anode stream passage; c) a cathode supply system (2) forsupplying an oxidant. stream to the cathode stream passage; d) acontroller (4) for operating one supply system (1, 2) in dependence onthe dynamic response of the other supply system (2, 1) characterized inthat the anode supply system. comprises a fuel processing system (1) forconverting a supply fuel into a hydrogen-rich gas wherein the controller(4) sets the hydrogen-rich gas mass flow ({dot over (m)}_(H2)) to theanode stream passage in dependence on the dynamic response of thecathode supply system (2).
 8. A fuel cell, system as recited in claim 7,wherein the controller (4) sets the supply fuel mass flow ({dot over(m)}_(Fuel)) to the fuel processing system (1) in dependence on thedynamic response of the cathode supply system (2).
 9. A fuel cell systemas recited in claim 7, wherein the controller (4) sets the oxidantstream mass flow ({dot over (m)}_(Air)) to the cathode stream passage independence on the dynamic response of the anode supply system (1).
 10. Afuel cell system as recited In claim 7, wherein the controller (4) setsthe oxidant stream mass flow ({dot over (m)}_(Air)) to the cathodestream passage in dependence on the dynamic response of the fuelprocessing system (1).
 11. A fuel cell system as recited in claim 7,wherein the controller (4) sets the supply fuel mass flow ({dot over(m)}_(Fuel)) to the fuel processing system (1) in dependence on theeffidency (η) of the fuel processing system (1).
 12. A fuel cell systemas recited in claim 7, wherein the controller (4) sets the supply fuelmass flow ({dot over (m)}_(Fuel)) to the fuel processing system (1) independence on the intrinsic consumption of hydrogen-rich gas in the fuelprocessing system (1).